1. Field of the Invention
Gold matrices generally used for jewelry include alloys such as 14-karat or 18-karat gold alloy, and Ni, Pd, Zn, etc. are added in large quantities to these alloys to increase their hardness or tensile strength. These alloys cannot therefore be called pure gold in respect of purity.
A high-purity gold alloy according to the present invention has a purity of 99.7% or more, and its hardness is increased to a level approximately equivalent to that of 18-karat gold at a relatively low working ratio by (1) adding trace elements and (2) performing a heat treatment in the process of a production process, thereby eliminating the drawbacks accompanying the enhancement of purity, that is, improving the workability, heat resistance, flaw resistance, etc.
2. Background Information
High-purity gold jewelry is low in hardness and it is extremely difficult to retain its aesthetic value for a long term in daily life. Also, a heat treatment performed during the production process, such as brazing, inevitably causes a great reduction in the hardness. The use of high-purity gold as ornaments is therefore limited.
Alloys obtained according to the present invention had a gold content of 99.8% or more and their Vickers hardness (Hv) was as high as 100 or more for cast articles and 150 or more for worked articles. Even with the use of compositions qualifying as pure gold, the hardness Hv was higher than 100 for cast articles and higher than 150 for worked articles (working ratio: 99.6%). In the case where a heat treatment was performed with Gd added, the pure gold according to the present invention was remarkably increased in hardness and also improved in heat resistance. The pure gold thus obtained is less liable to be marred or scratched and undergoes less variation with time, and reduction in the hardness due to a heat treatment such as brazing is small.
To obtain high-purity hardened pure gold capable of retaining a high-quality look for a long term, research was conducted and as a result, an alloy with high hardness was obtained which contained 99.7% by weight or more of gold, to which was added 50 ppm or more of Gd as an alloying component, along with another element so that the total amount of the additional elements was 100 to 3000 ppm. Reduction in the hardness of this member due to heat treatment was small. Adding a smaller amount of the elements resulted in lower hardness, and the hardness was nearly proportional to the tensile strength.
As the heat treatment for obtaining the above high-purity gold alloy, solution heat treatment, rapid cooling and aging treatment were performed. The resulting alloy was less lowered in hardness by welding, brazing or the like and thus can retain high aesthetic value for a long term, proving to be suitable as an alloy for use as high-purity gold jewelry.